Personality disorders form a class of mental disorders that are characterized by long-lasting rigid patterns of thought and actions. Because of the inflexibility and pervasiveness of these patterns, they can cause serious problems and impairment of functioning for the persons who are afflicted with these disorders.

Personality disorders are seen by the American Psychiatric Association as an enduring pattern of inner experience and behavior that deviates markedly from the expectations of the culture of the individual who exhibits it. These patterns are inflexible and pervasive across many situations. The onset of the pattern can be traced back at least to the beginning of adulthood. To be diagnosed as a personality disorder, a behavioral pattern must cause significant distress or impairment in personal, social, and/or occupational situations.

B. The enduring pattern is inflexible and pervasive across a broad range of personal and social situations.

C. The enduring pattern leads to clinically significant distress or impairment in social, occupational, or other important areas of functioning.

D. The pattern is stable and of long duration and its onset can be traced back at least to adolescence or early adulthood.

E. The enduring pattern is not better accounted for as a manifestation or consequence of another mental disorder.

F. The enduring pattern is not due to the direct physiological effects of a substance or a general medical condition such as head injury.

People under 18 years old who fit the criteria of a personality disorder are usually not diagnosed with such a disorder, although they may be diagnosed with a related disorder. Antisocial personality disorder cannot be diagnosed at all in persons under 18.

The DSM-IV also contains a category for behavioural patterns that do not match these ten disorders, but nevertheless have the characteristics of a personality disorder; this category is labeled Personality Disorder NOS (Not Otherwise Specified). The previous version of the DSM also contained the Passive-Aggressive Personality Disorder and the Self-Defeating Personality Disorder. Passive-Aggressive Personality Disorder is a pattern of negative attitudes and passive resistance in interpersonal situations. Self-defeating personality disorder is characterised by behaviour that consequently undermines the person's pleasure and goals. These categories were removed in the current version of the DSM, because it is questionable whether these are separate disorders. Passive-Aggressive Personality Disorder and Depressive personality disorder were placed in an appendix of DSM-IV for research purposes.
Sadomasochistic personality may be included here.

The DSM attempts to represent a consensus view of the members of the American Psychiatric Association. However, more so than in other parts of the DSM, the classification of Axis II personality disorders—deeply ingrained, maladaptive, lifelong behaviour patterns—has come under sustained and serious criticism from its inception